Metaphysical poetry contains a metaphysical conceit. It compares two things so unalike that it takes a explanation to see the connection. With "The Flea," the comparison is between the speaker's love for the young lady and a common flea. At first glance, there is nothing romantic in it, but on closer inspection the comparison is more understandable. In stanza one, the speaker says that the flea has bitten both himself and his would-be lover. Their blood is already mixed within his body--so she can't say that it is a sin to mix their blood the normal way since the flea is already enjoying what the two people might and the flea didn't have to woo her first. Stanza two, we understand that the young lady has indicated she'd kill the flea then and be done with this saucy talk. The speaker begs for the the flea's life since, in killing it, she would kill not only their love, but the "unborn child" within the "walls of jet"--the flea. Kill me, he says, but don't add killing yourself and our "baby" to that list. In Stanza three, we come to understand that she does kill it. The speaker calls her action "cruel and sudden" since the flea was not guilty of anything; it was innocent except for the act of sucking the blood of both people. He comes back to the question of their intimacy by saying that nothing has happened with the death of the flea, so what harm could come from shared tenderness behind closed doors?